This invention relates to bistable regenerative feedback circuits in general (e.g., bistable pulse generator circuits), and to high-speed DC-coupled ECL (emitter coupled logic) Schmitt trigger circuits in particular.
An early transistorized bistable regenerative feedback circuit is shown, for example, in FIG. 1A. Refer also, for example, to FIG. 5.21(a) in Introduction to Integrated Circuits by V. Grimich and H. Jackson, page 287, McGraw-Hill, New York, 1975. Typically in the operation of such a circuit, transistors Q.sub.1 and Q.sub.2 become saturated, causing undesirably slow switching.
To improve switching speed, extra circuit elements such as resistive divider elements R.sub.B1 and R.sub.B2 were typically added to the circuit, as shown for example in FIG. 1B. The addition of extra elements R.sub.B1 and R.sub.B2 substantially eliminated the saturation problem associated with Q.sub.1 and Q.sub.2, but caused loss in feedback signal to Q.sub.2. To substantially reduce or eliminate this loss, a current source element was typically added to the circuit replacing resistor R.sub.B2, and an emitter-follower transistor Q.sub.3 was often added replacing resistor R.sub.B1 as shown, for example, in FIG. 1C.
To further improve switching speed and to obtain common mode rejection, the single-ended configuration of the circuits, as shown in FIGS. 1A-C, was often replaced by a differential configuration as shown in FIG. 1D. In this differential configuration, the current source resistor R.sub.E of the Schmitt switching pair Q.sub.1, Q.sub.2 (FIG. 1C) was replaced by a true current source (I.sub.E2), and additional resistors R.sub.L and transistors Q.sub.4, Q.sub.5, Q.sub.6 were added to the circuit, the transistors Q.sub.5, Q.sub.6 being linked in a wired-OR fashion to transistors Q.sub.3 and Q.sub.4 as shown in FIG. 1D. Replacement of resistor R.sub.E by a true current source ensured equal output signals at both the collectors of Q.sub.1 and Q.sub.2, and the addition of transistors Q.sub.5 and Q.sub.6 in the wired-OR fashion between the input signal and the Schmitt feedback signal enabled the input signal and the regenerative feedback signal to be combined at the input or base of the Schmitt switching pair Q.sub.1, Q.sub.2.
To achieve faster switching without saturation and without loss in feedback signal, it is seen from the foregoing that the practice has been not only to add elements such as resistive divider and current source elements but, also, to link certain of the transistors (e.g., transistor Q.sub.4 with Q.sub.5, and Q.sub.3 with Q.sub.6) so that they perform a wired-OR function between the source or input signal (i.sub.in and i.sub.in) and the Schmitt state (i.e., the conducting state of either Q.sub.1 or Q.sub.2). As a consequence, signal transistors (at the input or base of the Schmitt switching pair) from a low or less positive voltage to a high or more positive voltage state occurs at a fast rate (due to low drive impedance furnished by one of the transistors in the wired-OR configuration operating in an emitter-follower manner), but signal transitions from a high to a low voltage state occurs at a significantly slower rate since the wired-OR transistors (Q.sub.4 -Q.sub.5, Q.sub.3 -Q.sub.6) become turned off (with no current flowing through their collectors or out of their emitters).
What is needed, therefore, is a circuit that would not only cost less to produce and be more compact than prior art circuits by requiring less rather than more elements to operate at a fast rate, but more importantly, a circuit that would combine the Schmitt state and input signal without turning off, thereby providing a switching capability that would be faster than that of prior-art trigger circuits such as the DC-coupled ECL Schmitt trigger circuit shown, for example, in FIG. 1D.